Sub-band coding of digital speech signals provides a relatively simple but efficient technique to reduce the bit rate required for digital transmission of speech. As is well known, a speech signal to be digitally encoded is sampled at a rate corresponding to twice its bandwidth and each sample is converted to a multibit digital code. Therefore, the transmission rate of the digital codes exceeds the sampling rate of the speech signal by a factor corresponding to the number of bits in each digital code. With sub-band coding, the spectrum of the speech signal over a short time interval is partitioned into a plurality of substantially lower bandwidth sub-bands so that each sub-band need be sampled at only twice the lower bandwidth. Since each sub-band contains less information than the entire spectrum of the speech signal, the encoding may be made more specific to the sub-band and the transmission rate for the sub-band may be reduced. Consequently, the overall bit rate of the multiplexed sub-band digital codes representing the speech and the resulting transmission rate are lowered without affecting the perceived quality of the decoded speech signal.
While a substantial bit rate reduction is achieved by known sub-band coding arrangements, some systems require a further reduction to accommodate limited transmission channels. Adaptive differential pulse code modulation or adaptive predictive coding have been employed to encode each sub-band such as described in U.S. Pat. Nos. 4,538,234, issued to Honda et al Aug. 27, 1985, and 4,622,680 issued to R. L. Zinser Nov. 11, 1986, to provide an even lower bit rate by taking advantage of redundancy reduction in the sub-bands. Such arrangements, however, make the coding apparatus more complex, more expensive and very sensitive to transmission errors. Since communication systems using radio type channels are characterized by bursts of errors, ADPCM or APC coding cannot be used effectively. An alternative is to employ only dynamic bit allocation such as described in the article "Sub-Band Coder with Simple Adaptive Bit-Allocation Algorithm: A Possible Candiate for Digital Mobile Telephony" by Tor A. Ramstad appearing in the 1982 Proc. International Conference on Acoustics, Speech and Signal Processing, pp. 203-207 to obtain a large bit rate reduction. Dynamic bit allocation by itself, however, is not sufficient to insure good speech quality at low bit rates, e.g., 12 Kbps. As a result, impaired performance is experienced.
U.S. Pat. No. 4,538,234, issued to Honda et al. Aug. 27, 1985, discloses an adaptive predictive processing system using adaptive bit allocation in which a speech signal is split into a plurality of bands. Each sub-band portion of the speech is subjected to adaptive predictive coding and adaptively changing quantization characteristics in the predictive coding to reduce quantizing noise. An average amplitude of the signal of each band is detected for each time interval to reduce the hardware requirements. Systems employing speech coding such as cellular telephone require very low bit rates to meet the requirements of limited bandwidth and to permit incorporation of error correction adapted to the burst noise characteristics of the channel. In such systems, adaptive predictive coding even with bit allocation may not provide sufficiently low bit rates and does not achieve robust performance in the presence of channel errors.
If only dynamic bit allocation is used to obtain robust performance, the limited number of bits allocated to the sub-bands results in some sub-bands having no allocated bits in particular time frame intervals. The phenomenon of no allocated bits for a sub-band occurring irregularly in the sequence of time frame intervals causes chirp-like artifacts in the decoded speech which detracts from the intelligibility of the decoded speech. It is an object of the invention to provide improved sub-band coding at very low bit rates with a reduction of the detracting chirp-like artifacts.